Safety equipment such as fire alarms, severe weather warning devices, roadway hazard signs, and railroad safety equipment (i.e., traffic control, grade-crossing protection, and fault detection devices) each require a power source which will drive the equipment even in the event utility power is lost. A typical prior art power supply utilizes a battery and a charging circuit linked to the local utility power line to maintain a near constant battery voltage. When the battery is new, the amount of stored energy is well known. However, as the battery ages, it is capable of holding less and less energy, and will eventually fail to meet the minimum standard for a safe and reliable power source. Because an undercharged battery in these power supplies can have fatal consequences by rendering the safety equipment inoperative if utility power fails, it is vital to periodically measure the stored electrical energy in these batteries and to replace the batteries before the minimum safe energy level is reached.
While voltage readings are helpful in determining weak battery cells in a poorly performing battery, measuring voltage alone is an unreliable charge indicator because battery cells may have a nearly constant voltage over a wide range of charge states. Several techniques are known in the art for measuring the state of charge in a battery. One of these techniques measures the specific gravity of the battery electrolyte. With rechargeable batteries (typically using an electrolyte of sulfuric acid and water), the chemical reaction accompanying the battery discharge creates water, reducing its specific gravity. The specific gravity of a fully charged cell is typically 1.300 and the voltage is typically 2 volts-2.4 volts. However, measuring the specific gravity requires access to the battery electrolyte. With older batteries, the electrolyte is in liquid form and the cell is covered by a removable cap. Removing the cap and measuring the specific gravity with the aid of a hydrometer is a common procedure. However, newer batteries use an electrolyte in gel form which is housed in a sealed fiberglass enclosure making it very difficult to measure the electrolyte's specific gravity. Thus, measuring the specific gravity is less feasible for determining battery charge in present field applications.
Another technique for measuring the charge of a battery includes removing the battery from service and connecting it to a charge tester which measures the charge by draining the battery well below the minimum safe energy level (typically 1.75 volts/cell). Because the battery is nearly discharged at this point, the battery must be recharged before being returned to service. This is the preferred technique in the prior art for determining if the battery can retain a minimum safe charge level.
Among other problems, the above prior art technique is expensive, time consuming, and often hazardous. In industries such as the railroad industry where thousands of power supplies are scattered over thousands of miles, many in remote and hard to reach areas, dispatching a crew to these locations is, in itself, quite expensive. A typical Class I railroad may have a power supply for traffic control, grade crossings, and/or fault protection devices every 1-4 miles over twenty thousand (20,000) miles or more of right of way. Once the crew reaches a given power supply, a temporary replacement battery must be installed, and the targeted battery must be tested. While the prior art technique typically drains the battery across a load 2-3 times the designed equipment load (in order to minimize testing time) it takes on average 8 to 12 hours to drain the battery down to 1.75 volts/cell. Moreover, because the battery is drained well below the minimum safe energy level, it must be recharged before being returned to the power supply--which takes an additional several hours.
As demonstrated, this prior art technique is time consuming and highly labor intensive which often requires multiple trips to very remote locations in order to test and properly recharge a battery. In fact, the test is so labor intensive and costly, that the required frequency of testing simply is not performed in many cases. This is especially true with geographically remote power supplies. The result of infrequent testing (or no testing at all in some instances) is an increased rate of battery failure during utility power outages thereby exposing the public and railroad crews to undesired and avoidable safety risks.
Another drawback to this prior art technique is the danger associated with accelerated discharging and charging of aging batteries. As mentioned above, to reduce the time necessary to test these batteries the prior art charge testing equipment utilizes a load 2-3 times the equipment load which the battery is designed to drive, thereby accelerating the battery discharge. The battery is likewise recharged at an accelerated rate. The increased current associated with accelerated discharging and charging tends to increase the temperature of any loose internal electrical connections and conduit thereby increasing the risk of battery explosions and the dangers associated therewith.
In order to overcome these and other problems in the prior art, the inventors herein have succeeded in designing and developing a power supply which can automatically measure a battery charge level without removing the battery from service and without accelerated battery discharging and recharging. In its simplest form, the power supply of the present invention includes a battery connected between an input power source and an equipment set, with a silicon controlled rectifier (SCR) controlled by a control circuit for switching the input power source off to test the battery. A battery voltage sensor and load current sensor are also connected to the control circuit to monitor the battery during testing. To measure the battery charge, the control circuit switches off the SCR thereby disconnecting the input power and causing the battery to discharge across the known equipment load until a minimum discharge voltage or a maximum discharge time is reached. At the end of the discharge mode, the control circuit automatically recharges the battery in an "equalized mode" and then maintains the battery at a programmed voltage level during a "float mode".
By selecting the minimum discharge voltage and maximum discharge time such that the battery charge is only partly drained during the discharge mode, the present invention avoids draining the battery below the minimum safe charge level such that it is always capable of responding to an emergency--even immediately after the test is complete. Therefore, the battery need not be taken out of service in order to measure the available charge which results in significant cost and labor savings. Moreover, because the discharge mode of the present invention does not increase the discharge load above that provided by the equipment which the power supply is designed to power, accelerated battery discharge is avoided. Likewise, the increased risk of explosion associated therewith is also avoided.
While in the discharge mode, the control circuit continuously monitors the battery voltage, ampere-hours drained from the battery, and the elapsed discharge time. When either the programmed maximum elapsed discharge time or programmed minimum discharge voltage is reached, the discharge parameters are recorded in non-volatile memory. As the battery charge is related to the time necessary for the known equipment load to drain the battery, a decrease in the time necessary to reach the minimum discharge voltage over the course of several discharge tests indicates a corresponding decrease in available battery charge. Thus, by tracking the discharge parameters (voltage, time, and ampere-hours discharged) the appropriate time to replace the battery is predictable.
Measuring the battery charge within the power supply of the present invention may be simply initiated by pressing an appropriate button or inputting an appropriate code to the control circuit. An operator may then return to the unit at a convenient time in the future to read the discharge parameters. This may be accomplished in a matter of minutes as opposed to the several hours (or even days) required in the prior art to remove the battery from service, install a temporary replacement battery, manually connect the targeted battery to a stand-alone testing unit, drain the battery, recharge the battery, and return it to service. With the present invention, once the discharge mode is initiated, the battery charge is tested and recharged automatically making it much more likely that the units will be regularly tested thereby providing greater assurance that the above mentioned safety devices will function properly in the event of a power outage.
Moreover, the control circuit may be programmed to automatically initiate the discharge mode at given time increments thereby further eliminating the need to dispatch a crew to manually perform this task. The power supply of the present invention may also include a transmitter and receiver to facilitate remote initiation of the discharge mode and remote retrieval of the discharge parameters thereby further reducing the labor associated with dispatching crews to remote locations.
Thus, the present invention satisfies a long-felt need by providing a power supply with automatic battery charge testing capability which is safe and simple to operate. While the principal advantages and features of the present invention have been briefly described above, a more thorough understanding and appreciation for the advantages and features of the invention may be attained by referring to the drawings and descriptions of the preferred embodiment which follow.